1. Technical Field of the Invention
The present invention relates to the recovery of nitric acid contained in a process stream comprising dinitrated aromatic compounds, and more especially, relates to the recovery and recycling of nitric acid contained in an organic stream principally comprising dinitrated aromatic compounds.
2. Description of the Prior Art
Processes for preparing aromatic dinitrated compounds have long been carried out on an industrial scale. These compounds are valuable intermediates for the preparation of aromatic diamines which themselves are used for synthesizing the corresponding isocyanates. The isocyanates are advantageously converted into polyurethanes, the applications of which are very extensive.
In brief, dinitration reactions are typically carried out in two steps, the first entailing preparing the mononitrated compounds and the second preparing the dinitro compounds. Other than the aromatic compound reactant, a nitrating acid is employed which is generally a nitric acid/sulfuric acid mixture, the sulfuric acid being a catalyst for the reaction.
After each nitration step, the dinitrated compound is separated from the residual acid. This operation conventionally is carried out by direct settling of the reaction mixture or by centrifuging the latter.
Apart from the mononitrated compounds used for synthesizing the desired dinitrated compounds, the dinitrated compounds obtained after separation together with the nitrating acid (also deemed "crude dinitrated compounds") cannot be used as such since they always contain, in the dissolved state, a fraction of the nitrating acid as well as organic impurities.
Solving the problems associated with the purification of aromatic dinitrated compounds, and more particularly with the separation and recovery of the dissolved acid, has been the subject of considerable research.
This is because, given the tonnage of dinitrated compounds produced annually, it is a not insignificant economic advantage to recover these residual acids. Moreover, their recovery also impacts the environment since such recovery has, in particular, the consequence of limiting aqueous discharge. In addition, recovering the acids makes it possible to reduce the necessary cost of treating waste water, which cannot be discharged as is since it is contaminated with salts, such as sulfates and, more significantly, nitrates.
The known technique for separating and recovering the dissolved nitric acid have all focused on increasing the effectiveness with which the dinitrated compounds are washed.
In the alternative described in EP-279,312, crude dinitrotoluene is washed with a very small amount of water. In this fashion, the wash water, laden with nitric and sulfuric acids, is sufficiently concentrated to be directly recycled into the nitration process. However, such a process presents the drawback of requiring a special apparatus for separating the aqueous phase from the organic phase, in this instance a coalescer. This is because settling is difficult due to the very low water content used for the washing. Furthermore, the extraction efficiencies are at most 72%.
The embodiment described in EP-736,514 entails washing, in several countercurrent steps, the crude dinitrotoluene with water laden with acids which are employed in the nitration reaction. The recovered water may be recycled into the nitration process, without prior concentration, or, preferably, with prior concentration. However, this process does not have all of the desired safety guarantees, since the step of concentrating the acidulated water, which proves to be necessary in most cases, presents not insubstantial risks. This because the distilled aqueous phase comprises both the nitrating acid and the dissolved dinitrotoluene. Consequently, the conditions are for the nitration of a dinitrotoluene, resulting in trinitrotoluene, the dangerous properties of which are well known to this art. Moreover, the dinitrotoluene and its isomers present in the aqueous acid phase emanating from the wash operation are not recovered in cases in which the concentration of said water is carried out. This because such compounds are entrained with the water during distillation, and are lost. Too, the dinitrated compounds may cause the concentration column to become blocked or fouled, since they condense at the top of the column and are then in a solid form. Likewise, they may be present in the water intended to be discharged, causing pollution or added cost if it is necessary to remove them. Finally, in order to have good nitric and sulfuric acid recovery efficiencies, it is necessary to employ two or three washing stages, in other words at least two or three mixer/settler units comprising, for each of the units, a step of recycling the aqueous phase. Consequently, the process is complex to implement and requires a significant amount of investment.